In the automatic transmission prior art, particularly for motor vehicles, comprised planetary gear sets switched by means of frictional or shifting elements such as clutches and brakes and were usually connected with a starting element subject to slip effect and optionally provided with a lock-up clutch such as a hydrodynamic torque converter or a fluid clutch.
Such a transmission results from EP 0 434 525 A1. It essentially comprises one input shaft and one output shaft parallel to each other, one double planetary gear set disposed concentrically with the output shaft and five shifting elements in the form of three clutches and two brakes, the selective locking of which by pairs determines the different gear ratios between the input shaft and the output shaft. This transmission has one front-mounted gear set and two power paths so that by the selective engagement by pairs of the five shifting elements, six forward gears are obtained.
In the first power path, two clutches are needed to transmit the torque from the front-mounted gear set to two elements of the double planetary gear set. These are situated in power flow direction essentially behind the front-mounted gear set in direction of the double planetary gear set. In the second power path, one other clutch is provided which detachably connects them with one other element of the double planetary gear set. The clutches are disposed in a manner such that the internal disc carrier forms the output drive.
The published U.S. Pat. No. 6,139,463 has further disclosed a compact multi-speed transmission in planetary design, particularly for a motor vehicle, which has two planetary gear sets and one front-mounted gear set, the same as three clutches and two brakes. In this known multi-speed transmission, two clutches C-1 and C-3 are provided in a first power path for transmitting the torque from the front-mounted gear set to the two planetary gear sets. The external disc carrier or the cylinder or piston and pressure compensation side of the clutch C-3 is connected with a first brake B-1. Besides, the internal disc carrier of the third clutch C-3 is connected with the cylinder or piston and pressure compensation side of the first clutch C-1, the internal disc carrier of the first clutch C-1 being situated on the output side and connected with a sun gear of the third planetary gear set.
From the Applicant's DE 199 49 507 A1, a multi-speed transmission is also known where, on the input shaft, two non-shiftable, front-mounted gear sets are provided which, on the output side, generate two rotational speeds which, together with the rotational speed of the input shaft, can be optionally shifted by selective closing of the shifting elements used to a shiftable double planetary gear set acting upon the output shaft so that, for changing over from one gear to the respective next following higher gear or lower gear of the two precisely actuated shifting elements, only one shifting element has to be engaged or disengaged.
DE 199 12 480 A1 has also disclosed an automatically shiftable motor vehicle transmission having three one-spider planetary gear sets, the same as three brakes and two clutches, for shifting six forward gears and one reverse gear and having one input shaft the same as one output shaft. The automatically shiftable motor vehicle transmission is designed so that the input shaft is directly connected with the sun gear of the second planetary gear set and that the input shaft is connectable via the first clutch with the sun gear of the first planetary gear set and/or via the second clutch with the spider of the first planetary gear set. Additionally or alternatively, the sun gear of the first planetary gear set is connectable with the housing of the transmission, via the first brake, and/or the spider of the first planetary gear set, via the second brake, and/or the sun gear of the third planetary gear set, via the third brake.
The problem on which the invention is based is to propose a multi-speed transmission of the type mentioned above in which the construction cost is optimized and, in addition, the efficiency degree is improved in the main drive gears with regard to towing and toothing losses. Besides, low torques are to act upon the shifting elements and planetary gear sets in the inventive multi-speed transmission and the rotational speeds of the shafts, shifting elements and planetary gear sets are to be kept as low as possible. The number of gears and the spreading of the transmission are also to be increased.